Caption: View of part of the inside of the IMB proton decay after it was completed in 1981. The tank contains 21 cubic metres of ultrapure water & is located 600 metres underground in the Morton salt mine near Cleveland, Ohio. The photo shows the bottom of the tank and one of the side walls. All sides of the tank are lined with photo-multiplier tubes on a 1-metre grid. The tubes detect & multiply tiny flashes of Cerenkov light produced in the tank by the passage of particles through the water or the decay of protons in the atoms of the water itself. The underground location reduces the unwanted effects of cosmic ray particles. (1981)